1. Field of the Invention
The present invention relates to a circuit board used various electronic apparatuses.
2. Description of the Related Art
With a recent trend for a smaller size and a lighter weight of electronic apparatuses and a higher functionality thereof, there have been desired for such a circuit board that is smaller in size, lighter in weight, and higher in signal processing capability as well as mounting density. The manufacturing of such a circuit board that can meet these desires requires technologies for increasing the number of layers, decreasing the diameter of a via hole, and finer patterning of the circuits implemented thereon. Therefore, those desires have been extremely difficult to satisfy by a circuit board on which interlayer electrical connections are implemented by the conventional through-hole construction.
To meet those desires, new circuit boards have been developed. The materials of the circuit board have also included, besides the conventional glass-based materials, organic fiber-based materials and films on a trial basis.
One representative of such circuit boards employs a full-IVH construction in which the interlayer connection is implemented by conductive paste (see Japanese Patent Publication No. 2601128). This circuit board uses a composite material such as aramid-epoxy resin to form insulator layers thereof, to have such advantages as a smaller coefficient of thermal expansion, a lower dielectric constant, and a lighter weight, thus finding wide application in a variety of electronic apparatuses.
To further improve the functionality of the circuit board, however, it is necessary to secure the stability in its connection resistance for further prolonged period of time.
In view of the above, it is a principal object of the present invention to secure a further prolonged stability of the connection resistance.
For the purpose of achieving the above mentioned object, in short, A circuit board of the present invention includes: at least two wiring layers; an insulator layer for electrically insulating the wiring layers; a via provided in the insulator layer to electrically connect the wiring layers each other; and a protective agent dispersed and placed in mottle-like on an interface between the via and the wiring layer to protect the wiring layers, wherein:
each dimension of the interface regions where the protective agent does not exist is set to such a dimension that a plurality of conductive powders constituting the via may abutted on the wiring layer; and
the plurality of the conductive powders and the wiring layers are abutted each other in each the interface regions where the protective agent does not exist to electrically connect.
With this structure, the following actions are demonstrated. In a region where the protective agent does not exist in an adhering region between the wiring layer and the via, the conductive powder constituting the via and the wiring layer are interconnected electrically. Moreover, in a region where the protective agent does not exist, since the plurality of the conductive powders abut on the wiring layer to be electrically interconnected, the electrical connection is further strengthened, thereby ensuring the prolonged retention stability and reliability of the connection resistance. Further, in a region where the protective agent exists, the wiring layer and the via can be prevented from invasion of water or oxidation by means of protection of a protecting layer.
In this case, it is preferable that the protecting layer is provided on the surface of the wiring layer. Therefore, it is relatively easy to provide the protecting layer on the interface.
The protective agent that can be used in the present invention may include substance that contains, for example, zinc or nickel.
Further, the protective agent can be made of such a material that enhances the adhesion strength between the via and the wiring layer to strengthen the physical adhesion therebetween, thus further improving the prolonged retention stability.
The protective agent that may improve the adhesion strength includes a substance containing a chromate or a silane coupling agent.
Further, when the conductive powder and the wiring layer are joined each other by metallic agglutination, a corresponding metallic bond develops therebetween, whereby a strong conductive path is formed to obtain good electrical connection. As a result, the prolonged retention stability and reliability of the connection resistance are further improved.
Further, if the size of the region where metallic agglutination develops between the wiring layer and the via is increased to 0.03% or more of the cross-sectional area of the via, the prolonged retention stability and reliability of the connection resistance can be further improved.
Further, if the via is made of a resin component containing a conductor, it can be shrunk by heating it under pressure, to thereby increase the ratio of occupation of the conductor component in the via, thus further securing the conductive path.
Further, if the conductor contained in the via is made of the same material as that of the wiring layer, the metallic bond or agglutination can be generated easily.
Such a circuit board can be manufactured by a method including the steps of: forming a through hole on an insulator layer and then filling the through hole with a conductive paste; dispersing and forming a protective agent in mottle-like on an adhesion surface of a conductor foil which provides a wiring layer, and dispersing and forming each dimension of adhesion surface regions where the protective agent does not exist in a state that the plurality of the conductive powders constituting the conductive paste is set to be capable of abutting on the said wiring layer; sticking the conductor foil to the insulator layer; and electrically and physically joining the conductor foil and the conductive paste by abutting the plurality of the conductive powders and the conductor foil each other by means of heating and pressurizing for the insulator layer.
Here, the protective agent is stored and placed into a minute recesses portion in the adhesion surface by abutting the adhesion surface of the conductor foil on a protective agent containing liquid, while a storage amount of the protective agent for the minute recess portion is controlled by adjusting abutting time of the protective agent containing liquid, thereby setting each dimension of the adhesion surface regions where the protective agent does not exist.
In addition, in accordance with a method of setting each dimension of the adhesion surface region where the protective agent does not exist, the protective agent is stored and placed into the minute recess in the adhesion surface by abutting the adhesion surface of the conductor foil on the protective agent containing liquid, while the storage amount of the protective agent for the minute recess is adjusted by adjusting containing amount of the protective agent of the protective agent containing liquid, thereby setting each dimension of the adhesion surface region where the protective agent does not exist.
Still further, a layer containing the protective agent is formed on the adhesion surface of the conductor foil, then, the protective agent layer is polished to such an extent that a top portion of minute protrusion on the adhesion surface may be exposed, while an exposed amount of the top portion of minute protrusion is adjusted at polishing, thereby setting each dimension of the adhesion surface region where the protective agent does not exist.